To examine the effect of blunting the rise in body temperature on exercise metabolism, seven endurance-trained men cycled for 40 min at 65% of maximal oxygen consumption (VO2,max) in an environmental chamber at either 20 degrees C and 20% relative humidity (RH) (T20) or 3 degrees C and approximately 50% RH (T3). The trials were conducted in random order at least 1 week apart. Mean oxygen consumption (VO2) during exercise was not different when comparing the two trials. In contrast, the mean respiratory exchange ratio (RER) was lower (P < 0.05) at T20 compared with T3. Heart rate, rectal temperature and plasma catecholamines were higher (P < 0.05) during exercise at T20 compared with T3, as was post-exercise muscle temperature (P < 0.01). Muscle and blood lactate and blood glucose concentrations were not significantly different when comparing T20 with T3. Net muscle glycogen utilization was greater (P < 0.05) at T20 compared with T3. These results suggest that glycogenolysis in contracting skeletal muscle is reduced during exercise when the rise in body core temperature is attenuated. These changes in carbohydrate metabolism appear to be influenced by alterations in muscle temperature and/or sympatho-adrenal activity.